In geological exploration it is desirable to obtain information regarding the various formations and structures which exist beneath the Earth's surface. Accordingly, various sensors, probes, and test equipment (collectively referred to as “tools”) have been employed to determine geological strata, density, porosity, composition, etc. when searching for hydrocarbon reserves. For example, dual detector compensated density tools, wherein a gamma source and two corresponding detectors or sensors are used to collect geological data, have been widely used for hydrocarbon exploration since the 1980's.
Dual detector compensated density tools are often employed in logging while drilling (LWD) operations, so as to provide information regarding the geological structures without requiring removal of the drill stem. In operation, the dual detector compensated density tool is used to make measurements in a plurality of azimuthhally binned sectors (e.g., 16 binned sectors) to produce a density log providing a 360° view around the borehole.
In a LWD dual detector compensated density tool configuration, the source and detectors are located on the side wall of the drill collar to avoid attenuation of the gamma rays by the drill pipe. Thus, the source and detectors are disposed off-center, or on an eccentric, with respect to the center of the borehole.
Although density images acquired using the foregoing LWD dual detector compensated density tools can reveal sedimentary structure of formation penetrated by the borehole, the effect of formation (e.g., sediment bedding) and borehole geometry on density measurement has heretofore not been addressed. For example, many post-processing procedures used with respect to dual detector compensated density tools assume a one-dimensional variation of the formation density. The widely used Δρ density compensation technique assumes an infinitely thick formation, and hence assumes a one-dimensional radial variation. Similarly, the commonly used α-processing technique, used for vertical resolution enhancement, assumes one-dimensional vertical variation.
Such one-dimensional assumptions have typically provided acceptable results with respect to vertical borehole geometries penetrating horizontal formation geometries (e.g., horizontal sediment beds). However, it has been discovered that such conventional density compensation post-processing procedures result in significant error, and the benefits resulting from α-processing decrease, as the relative dip between the borehole and formation increase. For example, high angle and horizontal (HA/HZ) wells, and similarly wells penetrating sediment beds having an appreciable bed dip, experience errors in bulk density (RHOB) estimation and bed boundary detection. The eccentricity and azimuthal rotation while drilling of the LWD dual detector compensated density tools make the tool response difficult to interpret with regard to the borehole and formation, particularly in HA/HZ wells.